The invention relates to a method for controlling a powertrain in a vehicle during acceleration. The invention involves limiting the vehicle acceleration by controlling the powertrain if the estimated time between the two sequential power shifts is shorter than a predetermined time limit.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as buses, construction equipment and other work vehicles.
Automatic and semi-automatic mechanical transmission systems for vehicles are well known in the prior art. Typically, such systems comprise a multi-speed mechanical change gear transmission, at least one friction clutch drivingly interposed the engine and the transmission and a central processing unit or controller for receiving input signals from sensors and issuing command output signals to actuators for operating the transmission. In a multi-clutch transmission such as a dual clutch transmission (DCT), power shifting may be preferred in most situations. During power shifting the gear shifts will occur sequentially and the clutches will be actuated alternately to give smooth gear changes.
Vehicles provided with multi-clutch transmissions are provided with a controller that will control the operation of the clutch based on a gear strategy stored in the control unit. The gear shifting strategy may under certain circumstances select a skip shift, e.g. between two even gears, requiring the same clutch to be used. Skip up-shifts can be demanded in situations when powerful vehicle acceleration is performed. During an acceleration the engine speed after a power up-shift can be higher than or near the next up-shift speed, leading to a short time between successive upshifts and an increase in number of gear shifts. The skip shifts are used in order to avoid a series of rapid single up-shifts by shifting directly into a desirable lower gear ratio.
However, skip shifts result in power cut-off gear changes which are less comfortable for the driver and may increase the risk of wheel spin. As a rule, drivers prefer to use power shifts as often as possible.
Hence it is desirable to provide an improved method for controlling a multi-clutch transmission that overcomes the above problem.
It is desirable to provide a method for controlling a multi-clutch transmission in a vehicle, which method allows skip up-shifts to be avoided during vehicle acceleration.
The invention relates, according to an aspect thereof, to a method for controlling a powertrain in a vehicle during acceleration. The powertrain comprises a propulsion unit, a multi-clutch transmission drivingly connected to the propulsion unit, and an electronic control unit for controlling at least the powertrain components. The electronic control unit is provided with a prediction model comprising at least one simulated shift sequence for the multi-clutch transmission. The multi-clutch transmission is preferably a dual clutch transmission.
The method involves performing method steps of:                monitoring at least one operating parameter of the powertrain;        estimating the time between initiation of a first power upshift and initiation of a sequential second power upshift using the prediction model; and,        
if the estimated time between the initiation of two sequential power shifts is shorter than a predetermined time limit,                controlling the propulsion unit to limit the vehicle acceleration so that the estimated time between sequential power shifts is increased to be at least equal to the predetermined time limit.        
The predetermined time limit is at least equal to the minimum time in which a power shift can be performed. This time is measured from the actuation of a first clutch, when a first power upshift is initiated, to the actuation of a second clutch, when a second power upshift is initiated. In this context, initiation of a first power upshift will involve actuation of a first clutch to engage the next gear in a sequence of gears, while at the same time deactivation of a second clutch to disengage a currently engaged gear. Similarly, initiation of a second power upshift will involve actuation of the second clutch and deactivation of the first clutch. This alternate clutch actuation is desirable for achieving smooth and uninterrupted sequential gear shifting in the multi-clutch transmission.
If the estimated time between initiation of a first power upshift and a sequential second power upshift is equal to or greater than the predetermined time limit then no action is taken and a power upshift to the next sequential gear is performed as normal. However, if the estimated time indicates that there is too little time for a power upshift between the estimated timing of the first and second gearshifts, then the gear strategy will carry out the final method step to limit the vehicle acceleration. This will delay the next power shift to allow the first power upshift to be completed before the second power upshift is initiated.
The predetermined time limit is preferably selected greater than the minimum time to perform a power shift. A longer time is selected to allow the transmission to run on each gear for a period of time to prevent excessive clutch slipping between consecutive clutch actuations and to avoid overheating of the clutches. A suitable predetermined time limit can be selected dependent on engine output power, vehicle size/load, clutch cooling capacity, or a similar suitable parameter. The predetermined time limit is also dependent on the time required by the transmission for preparing for the next power shift.
A limitation of the acceleration is preferably performed prior to the initiation of the first power upshift. Alternatively the limitation of the acceleration may be performed during a power upshift. This latter case will result in a longer time on the current gear due to the fact that the propulsion unit speed will be lowered immediately after the upshift. However, it will also give lower comfort as a part of the torque will be reduced during the gearshift. It will be something between a power cut-off shift and a power shift when it comes to torque reduction during the shifts. According to a further alternative, a combination of these two methods can be used, wherein the limitation of the acceleration is initiated prior to the initiation of the first power upshift and completed during the upshift. In all these cases the limitation of the acceleration will cause a delay to provide sufficient time for a power upshift, in order to avoid a skip shift.
The invention, according to an aspect thereof, is applicable to propulsion units comprising an internal combustion unit, a motor/generator, or a hybrid electric propulsion unit combining an internal combustion unit and a motor/generator connected to the multi-clutch transmission. When the propulsion unit comprises an internal combustion unit, the acceleration can be limited by injecting less fuel into the propulsion unit. When the propulsion unit comprises a motor/generator, the acceleration is limited by reducing electric power to the propulsion unit. Finally, when the powertrain comprises a hybrid electric propulsion unit, the acceleration is limited by driving an electric motor/generator using an internal combustion unit to regenerate energy. The regenerated energy can be stored in a battery, a super capacitor, or in any other suitable accumulator or energy storage device.
A reduction of the torque generated by the propulsion unit will reduce the input torque available to the multi-clutch transmission, thereby limiting the acceleration of the vehicle. The predetermined time limit is equal to the minimum time required for performing a power shift sequence. The control unit will use the prediction model comprising at least one simulated shift sequence for the multi-clutch transmission to calculate the output power reduction required to limit the vehicle acceleration so that the time between two sequential power upshifts is increased to be at least equal to the predetermined time limit.
The method can further involve monitoring at least one powertrain operating parameter comprising one or more of propulsion unit speed (rpm) and/or vehicle speed, using suitable sensors connected to the control unit. Further operating parameters can include an accelerator position, indicating an acceleration request from the driver, and/or road information, such as a current road gradient, which information can comprise GPS data stored on-board in the vehicle or data supplied from a centralized system.
The invention also relates, according to an aspect thereof, to a transmission control system comprising a control unit arranged for controlling a multi-clutch transmission in a vehicle, wherein the control unit is programmed to perform the above-mentioned method steps.
The invention further relates, according to an aspect thereof, to a computer program comprising program code means for performing the steps for controlling a powertrain in a vehicle when said program is run on a computer.
The invention further relates, according to an aspect thereof, to a computer readable medium carrying a computer program comprising program code means for performing the above-mentioned method steps for controlling a powertrain in a vehicle when said program product is run on a computer.
The invention further relates, according to an aspect thereof, to a control unit for controlling a powertrain in a vehicle, the control unit being configured to perform the above-mentioned method steps.
The invention, according to an aspect thereof, aims to achieve more power shifts under driving conditions when high acceleration is requested. The inventive concept involves increasing the time between upshifts by limiting the torque applied on the wheels, thereby limiting the acceleration, either by limiting the output torque from the propulsion unit or by using a portion of the output torque for regenerating energy, e.g. for charging batteries or super capacitors.
The invention, according to an aspect thereof, has the advantages of improving driveability and providing a more constant acceleration, resulting in a more constant torque on the wheels and less risk for slipping.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.